1. Field
The present disclosure relates to a techniques for cooling computer systems. More specifically, the present disclosure relates to techniques for generating control signals for a fan in a computer system based on a determined cooling rate of the computer system.
2. Related Art
Convective heat transport mechanisms, such as heat transported by flowing air, are often used to remove the heat generated by components in computer systems. In order to properly regulate the temperature in these computer systems, the rate of heat transport, as indicated by the air-flow rate, and more generally, the mass flow rate (e.g., in cubic feet per minute or CFM) needs to be determined to ensure that it matches or surpasses the rate of heat generation.
However, it is often infeasible to measure the air-flow rate in a computer system. In particular, sensors that directly measure the air-flow rate often have poor reliability and typically require frequent recalibration (which is economically prohibitive in many applications). As a consequence, cooling-control techniques in many computer systems are often based on dual ‘proxy variables’ for the air-flow rate. For example, a measure of the mean voltage applied to a fan motor (such as pulse-width-modulation or PWM metric of a fan-motor power signal) is often used as a proxy variable for the fan's rotation speed (in revolutions per minute or RPM). In turn, the fan's rotation speed is typically used as a proxy variable for the air-flow rate. This ‘dual proxy’ relationship typically requires two separate calibration processes, each of which is subject to possible errors.
For example, a PWM metric may be calibrated against the fan RPM to determine a PWM-vs.-RPM curve that is specific to a given type of fan in a given computer system. However, this curve is often strongly dependent on the mechanical integrity of the fan motor, shaft and blades. In particular, the PWM-vs.-RPM curve is typically determined when a fan is new (i.e., the fan motor is new, the bearings are as round as they will ever get, the shaft is well balanced and centered with little or no rotational eccentricity, the lubrication levels are optimal, etc.). As the fan ages, wear and other changes will increase friction levels, which make the previously determined PWM-vs.-RPM curve inaccurate. These inaccuracies can adversely impact the cooling capability and the temperature regulation in a computer system that includes the fan.
Similarly, a calibrated RPM-vs.-CFM curve may be determined by measuring air flow in the computer system using an instrumented flow chamber at a particular altitude. However, because of barometric-pressure and altitude variations, the resulting curve may be inaccurate. To address this problem, many computer systems are calibrated at their maximum rated operating altitude (for example, 10,000 ft.). As a consequence, these computer systems are usually overcooled at lower altitudes. While this approach is safe, it wastes energy and results in errors during temperature regulation.
In addition, the RPM-vs.-CFM curve of a computer system may change over time because of air-flow impedance changes, such as flow inhibitors or air-flow ‘short circuits’ in the computer system. Typical flow inhibitors include: buildup of dust on air filters, changes to the system configuration (such as adding, removing or moving internal components in the computer system), and/or unintentional obstructions (such as paper or foreign objects accidentally left inside of the computer system). Moreover, air-flow ‘short circuits’ can occur when an operator leaves an access panel in the computer system open.
Consequently, the use of proxy variables to determine the air-flow rate is often subject to errors, which increases the probability of overcooling (which wastes energy, and increases acoustic noise and vibration) or undercooling (which accelerates reliability problems, and can lead to down-time or temperature-induced shutdown of the computer system).
Hence, what is needed is a technique for determining the air-flow rate and providing accurate control signals to a cooling fan in a computer system without the above-described problems.